The present invention relates to a process for the suspension polymerization of vinyl chloride, and particularly to a novel process for the batchwise suspension polymerization of vinyl chloride, which provides a substantial improvement in the productivity, and wherein the polymerization is carried out in such a manner that after the monomer conversion has reached a certain level, at least a part of the polymerization reaction mass formed is circulated to an external cooling zone which is provided separately from the polymerization reaction zone.
Heretofore, the suspension polymerization of vinyl chloride has been carried out in such a manner that water, a vinyl chloride monomer, a polymerization initiator, a dispersing agent and, if necessary, an additive or additives, are fed into a polymerization vessel equipped with a jacket, through which cooling water is circulated to remove the heat of polymerization and thereby to maintain the reation temperature at a predetermined level.
The productivity per volume of the polymerization vessel may be increased by increasing the amount of the monomer fed into the reactor, and by properly selecting the type and amount of the polymerization initiator, it is possible to perform the polymerization reaction uniformly and thereby to shorten the polymerization time. In any case, however, the polymerization reaction is vigorously exothermic, and in order to remove the generated heat by the cooling water circulated through the jacket, the amount of the monomer to be fed and the amount of the initiator to be used must be limited to a certain level.
Accordingly, it is necessary to improve the cooling efficiency for removal of the heat generated by the polymerization reaction in order to improve the productivity. It has been proposed and industrially adopted to carry out the cooling by means of a refrigerator in addition to the cooling water jacket, thereby to increase the cooling efficiency. However, this method is economically disadvantageous. It has also been proposed and industrially in use to provide a condenser at an upper portion of the polymerization vessel, thereby enhancing the cooling efficiency. However, the above-said method is dependant on the evaporation, cooling and reflux of the monomer itself and the cooling effect of such system is limited. Besides, the product obtained by the method is of a poor quality and particularly it tends to suffer from increased fish eye formation.
Accordingly, it is an object of the present invention to eliminate the above-mentioned drawbacks and to present an industrially advantageous process for the suspension polymerization of vinyl chloride, in which, as a heat removal means which is fundamentally different from the conventional cooling means, an external circulation cooling of the polymerization reaction mass is employed, whereby the productivity per volume of the polymerization vessel can substantially be increased.
Generally, circulation of the reaction mixture to an external cooling apparatus is a very efficient method for removal of the heat from the reaction mixture, since it is thereby possible to enlarge the heat transfer surface area for contact with the reaction mixture. In the case of the polymerization reaction of vinyl chloride, however, polymer build-up tends to occur on the inner wall of the external cooling apparatus and on the inner surface of the circulation pipe line, as well as on the surface of the inner wall of the polymerization vessel, and such polymer build-up tend to clog the pipe line whereby it becomes difficult to control the temperature and thus they are likely to cause a serious hazard. For this reason, an external cooling method has not been industrially employed in the polymerization of vinyl chloride.
Further, the quality of polyvinyl chloride (hereinafter referred to an "PVC") depends primarily upon the morphological properties of the polymer particles, and it is likely that the morphological properties of the polymer particles be substantially deformed by the external circulation of the polymerization reaction mixture. Thus, the difficulty in the quality control of the polymer particles has been one of the reasons for not employing the external circulation cooling method in the conventional process for the suspension polymerization of vinyl chloride. Recently, a method for the suspension polymerization of vinyl chloride was disclosed wherein an unusually larger amount of a dispersing agent is used while the monomer is used in a smaller amount, and thus the slurry is circulated to an external cooling apparatus at a lower concentration (B.P. No. 2,001,659). In the method, however, the amount of the dispersing agent used is substantially greater than the normal method and accordingly the method brings about a product of extremely poor quality; namely, the products tend to be poor in plasticizer absorption property, gelation property, thermal stability, color transparency, and electrical insulation property and to suffer from fish eye formation. Further, the amount of the monomer used and accordingly the productivity per batch are smaller than usual. If the amount of dispersing agent were decreased to obtain a polymer of improved quality, polymer build-up would then occur on the inner surface of the external cooling apparatus and on the inner surface of the circulation pipe line even when an inhibitor for polymer build-up would be used, thus creating a serious disadvantage. Such polymer build-up would be even greater particularly when the amount of the monomer used is increased to a volume ratio of the monomer to water of 0.8 to 1.3 so as to increase the productivity.